Process of separating vinyl xylenes from ethyl xylenes and other products



Aug. 19, 1952 PROCESS w. E. ELwELl. ErAl. 0F SEPARATING VINYL XYLENESFROM ETHYL I XYLENES AND OTHER PRODUCTS Filed July 1, 1948 Patented Aug.19, 1952 PROCESS F SEPARATING`VINYL XYLENE FROM ETHYL'y `XYLENES ANDOTHER PRODUCTS William Elwell, Berkeley, vand ','Herm'an L.

Childress, Jr., Richmond, Calif., assignors to i yCalifornia ResearchCorporation, San .Francisco, Calif., a corporation .of .DelawareApplicationJuly 1, 1948,-Ser-ial No. 36,357A

1 Claim.

This invention relates to a .process for the distillation purication ofvinyl Xylenes and .re- Afers..particularly to a process forv purifying`,vinyl xylenes -from .other products, .which vinyl Xylenes tenuto.polymerize levenin thepresence of poly- Amerization inhibitors, .at thetemperatures obtained in commercial .distillation equipmentsunderordinary, economical distillation 'p'ressures .such as, for example,20e-,50 mm. .Hg..absolute.

:The .separationo ethyl rbenzene from the. .vinyl benzene wor, vstyreneis commonly effected commercially 'bydistillationfthrough the Aadditionof sulphur tothe 'distillation'. column to actyas an ,inhibitortovprei/ent polymerization of Vthe -viny1 vbenzeneor-styrene during thedistillation process. In `such a process, the te-mperaturethat isrealized lin the fdistillation, pot or. reboiler for 1a ,givenpressure-at the vacuum pumpsl isl dependent upon the pressure drop inthe system. Commonly, in the separation of Vethyl Ybenzene and styreneabout a 'rplatevcolumn .is required to give the desired degree ofseparation. If a pressure of 20 mm. Hg.V absolutenis obtained at thevacuum ipumps and la pressure drop of l mm. occurs in the condenser,and12 mm` Yper plate in the column, the pressure in' the mt or` reboilerwill be about 170mm. .I-Ig.1 ForY styrene this corresponds to a boilingytemperature of about8v5 to 90 C., which temperature has been foundsuiciently flowto inhibit polymerization, provided sulphur is utilizedas a polymerization inhibitor.

When attempts are made to apply such a process to the separation ofvinyl Vxylenes from ethyl xylenes it is not found possible to mainlow.as .to prevent the polymerization of the vinyl xylenespnless pressuresare. employed., at the pumpnwhich` are in the nature-fof extraordinarylow vacuums, which are not achievable'economically by presentknowncommercial equipment. It has been found, forl'example, that when areaction product which..analyzed about 20.to 130% vinylxylene, .theremainder being substantially entirely ethyl xylene, was subjected todistillation in a column with pump pressures as low as about mm. Hg.,that as much as 50% of the vinyl xylene was polymerized, even though thecolumn was maintained saturated with sulphur to inhibit polymerization.

We have discovered that it is possible to eect a purification of vinylXylene using pump presfsures obtainable in commercial practice such as20 to 50 mm. Hg. absolute, provided -steam is introduced into the pot orreboiler of the aptain the pot or reboiler temperature -sumciently'paratusfor the purpose of inhibiting polymerizati'onand.,-f,acilitating vaporization of the constituents. .1f Suche processis to be effected, it must becarried o ut in `rconjunction with thesatura- .tion..of..ithe column with sulphur to assistininhibitingpolymerization. In otherV words, we .have .found that the useof steam during vacuum .distillation `and the presence of sulphur `asAan yinhibitor iseliective in ,permitting distillation separation ofvinyl -xylene products, which otherwise Yundergo, vvvery excessivepolymerization. yEach of the three expedients.mentioned,`i. e., :vacuum.distillation` steam, and the use of sulphur; as an inhibitor, appearsnecessary in order .that distillationseparation be effected withoutpolymerization. Attempts made to effect separation by Avacuumdistillation alone, or using sulphur :and yacuum, or other inhibitorsand vacuum, result in excessive polymerization. Similarly, fattempts to,effect distillation through the .use of `steam vand vacuum eitherwithout any inhibitoryfor polymerizationor through the use of otherinhibitors of polymerization such as hydroquinone, finfplace of sulphur,areineiiective. '.By the addition of steam, vacuum and sulphur Vto thecolumn, it is possible to effect distillation at :temperatures as low as-about '75 C. toj8r9 and obtain 90%, orbetter, yield of vinyl vxylene,withoutv polymerization.

.Theprocess ,of the present invention will `rrbe Afully understood. fromthe following description of xa lpreferred lexample of vthe processasillus- 4trated;i n ther ,accompanying drawings.'

lnthedrawings, the :figure represents a dia- -granrimaticvviewof lanapparatus inwhich the `process Umay be gconducted, in which drawing,however, for -the sake of'clarity and simplicity, there have beenomitted certain details such as pumps,- valves,A measuring means, heatexchangers, coolersf-reluxing apparatus, flow meter-s, and likeappurtenances, as lwill be readily supplied by one skilled in the art. YY, j

n-the process-as illustrated in the drawings, .the Ivinyl. .xyleneproduct which contains Agenerally, in addition to vinyl xylene, ethylXylene and higher boiling point products such as tar (and it may alsocontain methyl indenes) is introduced through a line l into thedistillation column 4. Simultaneously, water (to be converted intosteam) or steam is introduced through line 2. The column 4 is generallyrequired to contain about '70 to 90 plates, and in the column thetemperature in the distillation pot or reboiler is maintained betweenand 90 C. Vacuum is applied by means of a vacuum pump 3 located 3 beyondthe condenser (not shown), in accordance with the usual practice, andoperates to produce a pressure at this point of about 20 to 50 mm. Hg.absolute. As a result of the pressure drops throughout the system andthrough the plates in the column, there Will be maintained a pressure ofabout 300 mm. Hg. at the pot or reboiler. A ratio of steam entering line2 to hydrocarbons entering line I' is maintained at about 2:1 to 5:1 byWeight.

In the column 4 the ethyl xylene is removed as an overhead through line6. but a portion thereof is recycled through line 'I to asulphursaturator 8, so as to re-enter the column 4 to maintain thecolumn saturated with sulphur. At the bottom of the column, through line9, there is removed the vinyl xylene, tar and methyl indene present, andthis material is introduced into the distillation column 5. The column 5is similar i'n construction to the column 4 except that generally alesser number of plates are necessary to eiect the vapor separationrequired, such as about 25 plates in the column. In this column again atemperature of around 75 to 90 C. is employed in the pot or reboiler.There is provided a vacuum pump l diagrammatically illustrated, butconnected as usual beyond the condenser (not shown) and operated tomaintain the same degrees of vacuum as is maintained in connection withVthe vacuum pump 3. Pressure on the pot or reboiler of the column shouldnot be higher 'than 300 mm. Hg. and may (in View of the lesser number ofplates) if desired, be maintained somewhat lower. The steam isintroduced through the line ll in a ratio of steam to hydrocarbons ofabout 2.5:1 by weight to assist in vaporization and inhibitingpolymerization. In column 5su1 phur saturation is maintained bycarry-over of sulphur from column 4 With the entering material throughline 9. From the top of column 5 the vinyl xylene is removed through aline l2 while the tars are separated through line I4.

As an example of the operation of the process, a product containingabout to 30 Vinyl xylene and about 2 -to Li1/2% methyl indene, theremainder being mostly the 2,5-dimethyl -l-ethyl b'enzene accompanied bysome tar, including polyethyl products, Was supplied to the distillationcolumn 4. This process'was a batch operation rather than a continuousprocess, and Water in a ratio of about 4 parts by Weight to one part ofhydrocarbons was supplied to the pot and distillationA separated at apot temperature of about '15"v C., which increased to 89 C. during therun. In the separation of the-vinyl xylene and tar, the recovery ofabout 95% of the vinyl xylene was effected. Y f l In the vinyl xylenevproduct obtained, only the rst portion of the product was freeA ofmethyl indene, as most of the methyl indene is Vaporized with the vinylxylene.

Where the feed material is free of methyl indene, a pure vinyl V'xyleneproduct may be ob- 4 tained. Thus, for example, a reaction product whichconsists of about 20 to 30% of vinyl xylene (substantially 85 to 95% ofwhich is 3,5-dimethyl l-vinylbenzene, the remainder-being mainly thecorresponding ethyl benzene with some tar) may be distilled to recoverabout 90% of the vinyl Y xylene as a pure product by distilling amixture v same with the substantial elimination of isomerization of thevinyl compounds during the process,

whereas before the introduction of steam, even though high vacum Wasemployed and sulphur inhibitor used, 50% or more ofthe product waspolymerized.

While the particular form of the process herein described is Welladapted to carry out the objects of the invention, various modicationsmay be made, and-this invention is of the scope set forth in theappended claim.

We claim: Y A process for separating a mixture of ethyl xylene, vinylxylene, and higher boiling products which comprises distilling themixture with steam in a rst distillation zone in the presence ofelemental sulfur to inhibit the polymerization of the vinyl xylene,under a reduced pressure of 20-50 mm. of mercury at the overhead outletof the distillation zone, and at a pot temperature below about C. toseparate ethyl xylene as the overhead fraction, distilling the bottomsfrom the iirst distillation zone comprising vinyl xylene, higher boilingproducts and residual sulfur in a second distillation zone under areduced pressure of 20 to-50 mm. of mercury at the overhead outlet ofthe second distillation zone and at a temperature below about 90 C. andintroducing about 2 to 5 parts by weight of steam per part ofhydrocarbon into the distilland during the distillation to separate anoverhead fraction consisting essentially of vinyl xylene.

WILLIAM F. ELWELL.

HERMAN L. CHILDRESS, J R:

REFERENCES CTED The following references are of record inthe ile of thispatent:

UNITED STATES PATENTS 578,965 Great Britain O Ct. 9, 1912

